Slopes, Hills, and How They Impact Water Pressure

Hills, Valleys, & Slopes:

“Consider the slope or you’ll look like a dope!”

Elevation changes can add or subtract water pressure from your water system. That seriously changes how well the system works. Each foot of elevation change is equal to 0.433 PSI of water pressure. Think of a vertical column of water. At the bottom of the column the weight of all the water above is resting on the bottom of the column, this weight creates pressure. Have you ever swam down to the bottom of a deep swimming pool and felt your ears pop or hurt? That’s caused by the increased water pressure pressing against your eardrum. The deeper you go, the more pressure you feel.

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Not Just for Irrigation

While this page is written for irrigation design, these same principles apply to any piping system that carries water in it. The elevation impacts described here would apply to a huge city water system, to the pipe bringing water from a well to a rural home, or a pipe taking water from a creek or pond to a remote tank. If you jumped here from an Internet search and are not working through the irrigation design tutorial this page is part of, remember that when designing a water piping system you must consider other sources of pressure loss in your design too, such as friction loss caused by the water moving through the pipe.

In the USA we measure water pressure most often in pounds per square inch (PSI). That’s the weight in pounds of the water on a one-square-inch surface area. Sometimes we measure pressure in “Feet of Head”, especially when dealing with pumps and wells. This is to confuse you. (Not really.) We also don’t use metric here in the good old USA. We do this to annoy the rest of the world. (No, we really do it because we are lazy and unwilling to adopt the metric system.) So if you are outside the USA water pressure is measured in bars… or kiloPascals (kPa). Or about a half dozen other measurements. Unfortunately the rest of the world is no more agreed on how to measure water pressure than we are! There are simply a lot of systems used to measure pressure. Fortunately a conversion calculator will allow you to switch back and forth between any of them. If you don’t like that calculator or it isn’t working there are many more, just search for “pressure conversion calculator.”

My tutorials mostly use PSI, although I use Feet Head in parts of the pump related tutorials and metric for drip systems. OK, now it’s class time!

Hydraulics 101

You can skip down past this section if you wish. Look for the next horizontal line. This section is for those who need to know “why?” or want to understand hydraulics.

Since water is essentially a non-compressible liquid it exhibits the unique trait of transferring pressure horizontally when in a confined space. What this means is that water in a pipe (which is a confined space) exhibits the same pressure as it would if the pipe were perfectly vertical, even if the pipe isn’t. This isn’t an easy principle to understand, so be patient and re-read as needed. The best way to demonstrate this is with a picture.

In this picture the water pressure in the water tank at the top of the water surface level is 0 feet of head, or you could also say there is 0 PSI. This is because there is no water above it to create pressure. Head is another word that indicates pressure, it is mostly used when measuring pressure created by the depth of water. So 10 feet deep water will create 10 feet of head at the 10′ deep level. So 10 feet of depth = 10 feet of head. Ok? (Yes, I know there would be a small amount of additional water pressure due to the air pressure above the water, but let’s try not to confuse things. This is hard enough to understand! So we’re going to say that there is 0 feet of head at the water surface.)

Looking again at the picture above, we see that the ground level is 40 feet below the water level in the tank. Therefore the water pressure at ground level is 40 feet of head. Again 40 feet of depth = 40 feet of head. Now lets convert that to pressure measured in PSI. As noted earlier, 1 foot of elevation change creates 0.433 PSI of water pressure. So in this case 40 feet of head is going to be about 17 PSI. (40 ft head x 0.433 psi/ft = 17.3 PSI.) Again, the formula is “feet of head x 0.433 = PSI.” So far, pretty straight forward. Read again if you’re confused.

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Static Water Pressure

Now the hard to understand part. In the drawing above, the water enters the house at a level 100 feet below the water level in the tank. So the static water pressure at the house is 100 feet of head, or about 43.3 PSI, using the formulas in the previous paragraph. Note that I said this is the “static pressure”. So now you’re likely wondering how this could be? The water level is not just 100 feet above the house there is also easily 180 feet of pipe between the tank and the house! The answer is that the length of a pipe does not matter when the water is static in the pipes. Static means the water is not flowing, it is not moving, it is standing still. This is very important! Because the water is a non-compressible liquid it transfers the pressure horizontally along the pipe route for pretty much any distance without any loss of pressure! Cool, right? You bet it is, it is a principle that is very handy and makes all sorts of neat gadgets used on machines work. This is why a small hose filled with hydraulic fluid can cause the brakes on every wheel of a mile long train to apply when the engineer hits the brakes!

Now on the other hand, if we measured the pressure with the water flowing, then the pressure would be termed “dynamic pressure”. With the water in a dynamic state (flowing in the pipe) the water would loose pressure due to friction on the sides of the pipe and we would get a lower pressure reading at the house shown in our previous diagram. (I’ll deal with dynamic pressure in the next paragraph.) So for now, just understand that static pressure means there is no flow in the system, so there is no friction, and no pressure loss! Read that last sentence again! Think about it for a second, go back look at the picture again if you need to. It makes sense if you think about it. Our professor spent a week drilling this concept into us back in college and a lot of people in the class never did understand it! So if you still don’t get it don’t feel bad and don’t get discouraged! Just accept it on faith (I wouldn’t lie to you) and continue on.

In most cases we use static water pressure values when designing irrigation systems (or any other water piping system for that matter.) Then we can use calculators, spreadsheets, or charts (if you really want to torture yourself you can even use a very complicated manual calculation) to estimate the “friction loss” that will occur in the pipes when the sprinkler system is operating. Then we will subtract the friction loss from the static pressure to arrive at the dynamic pressure. Why not just turn the water on and measure the dynamic pressure with the water flowing? It would seem simpler, then we would not have to prepare a separate calculation for friction loss, right? Well, that is correct, however dynamic pressure is extremely difficult to measure accurately! You have to get the flow just right, and then hold the flow at that level for a minute or two while the pressure stabilizes. This is a real pain in the rear to do and not nearly as easy as it sounds! Plus, it is a bit hard to do if the pipe isn’t installed yet! You can’t measure the dynamic pressure if the pipe isn’t installed! So, the result is that we almost always will work by using static water pressure and then use calculations to determine the dynamic pressure. Its just way easier to do, and who wants to do it the hard way?

Now go back and look at that picture at the top of this page of the tank and house again. As the water flows to the house the water level in the tank will go down (assuming water isn’t flowing into the tank to refill it.) So the elevation of the top of the water in the tank will drop as the tank empties. When the tank is almost empty the difference might be only 95 feet. So since the water depth is less, the water pressure would also be lower. This happens all the time and is normal! If the top of the water elevation varies, then the water pressure will also vary. So if the water level will vary at your water source, the pressure will also vary. I know I keep saying the same things over and over in different ways, but I’m trying to drive home some important, but hard to understand, principles! My apologies if you got it the first time through and are getting bored!

Still confused? Don’t worry about it, just follow through the procedures that follow and you’ll be all right even if you don’t fully understand why you’re doing some of these things! Just remember that whenever you measure water pressure with a gauge you need to turn off all the water outlets so the water is static, that is, not flowing.

Time to wake up!

Hills, Valleys, & Slopes Continued…

In a nutshell: Just remember every foot of elevation change causes a 0.433 PSI change in water pressure. If your pipe is going downhill add 0.433 PSI of pressure per vertical foot the pipe goes down. If the pipe is going uphill subtract 0.433 PSI for every vertical foot the pipe goes up. The word “vertical” is critical. If the pipe goes up a slope the vertical distance is how high the slope would be if the pipe were going straight up. Do not use the length of the pipe, use the change in elevation! If you don’t want to accept my word for it then you’re going to have to go back and read all that boring Hydraulics 101 stuff above!

Because elevation changes effect the water pressure we must take this into account when determining pressure loss in our water system. If the area to be irrigated is lower than the water source we will gain pressure, so we may be able to gain some beneficial added pressure to our system. Care must be taken though. We can only add pressure if ALL the irrigation system is lower. If portions of it are not lower, or are higher than the water source, then those portions aren’t going to be getting that extra pressure. It is safest when doing initial design work to just not add pressure for elevation changes unless you’re really sure.

On the other hand if portions of the irrigation system are higher than the water source you will always need to subtract out the pressure loss created by the elevation gain. Pressure gained can be easily disposed of, pressure lost however, is very difficult to replace. So, for every foot of elevation gain (higher) in the irrigation system, you should subtract 0.433 PSI from the design pressure.

Example:

The far corner of the irrigation system is 9 feet higher than the water source.

9 feet X 0.433 PSI = 4 PSI loss (loss because it is higher). The water pressure in the far corner will be 4 PSI lower than the pressure at the water source, simply because it is 9 feet higher.

Another example:

One corner of the irrigation system is 20 feet lower than the water source. Another corner is 12 feet higher than the water source. 12 feet X 0.433 PSI = 5 PSI loss at the higher corner. However in the 20 feet lower corner the pressure will be higher. 20 feet x 0.433 PSI = 8 .7 PSI higher in the lowest corner. In some cases we might need to install a device to lower the pressure at the sprinklers in that low corner. But we’ll worry about that later. For now the high corner with the 5 PSI loss is more important. Remember, it is easy to lower the pressure if we need to, but it is hard to raise it.

A final example:

The water source is on a hill. The highest part of the irrigation system is 50 feet lower than the water source. The lowest part of the irrigation system is 60 feet lower than the water source. In this case you can add pressure because the ENTIRE irrigation system is lower. But the pressure added can only be the difference between the water source and the highest part of the irrigation system. 50 feet x 0.433 PSI = 22 PSI pressure GAIN. So you would subtract this amount from the total system pressure required. In other words you would enter a negative number in your Pressure Loss Table for Elevation Pressure Loss.

Too Much of a Good Thing:

What if one corner of the irrigation system is a lot lower than the other? While unusual, it is possible to have too much pressure! With too much pressure the sprinkler heads might not work as well, or they might even blow apart! For spray type sprinklers 40 PSI at the sprinkler head is the most pressure you want. For rotors it varies, but most small systems shouldn’t have more than 70 PSI at the rotor sprinkler head. If you have too much pressure you will need to reduce the pressure. Most sprinkler heads can be bought with a built in pressure reducing device. You can also buy an individual pressure reducing device that can be installed on the sprinkler head inlet pipe. These devices will reduce the water pressure to the optimum level for the sprinkler. Remember, the devices only reduce pressure, they can’t increase it! They will always reduce the pressure by at least a small amount, so they should not be used unless you have too much pressure. More on this topic will be covered when we get to discussing sprinkler heads so don’t worry about it now.

If you are working on a Sprinkler Design enter the pressure loss or gain caused by elevation changes on the “Elevation Change” line of the Pressure Loss Table. Enter the value in PSI. Remember, feet of elevation change x .433 = PSI.